1. Field of the Invention
The invention relates to the field of static charge neutralization apparatus using corona discharge for gas ion generation. More specifically, the invention is directed to producing clean ionized gas flows for charge neutralization in clean and ultra clean environments such as those commonly encountered in the manufacture of semiconductors, electronics, pharmaceuticals and similar processes and applications.
2. Description of the Related Art
Processes and operations in clean environments are specifically inclined to create and accumulate electrostatic charges on all electrically isolated surfaces. These charges generate undesirable electrical fields, which attract atmospheric aerosols to the surfaces, produce electrical stress in dielectrics, induce currents in semi-conductive and conductive materials, and initiate electrical discharges and EMI in the production environment.
The most efficient way to mediate these electrostatic hazards is to supply ionized gas flows to the charged surfaces. Gas ionization of this type permits effective compensation or neutralization of undesirable charges and, consequently, diminishes contamination, electrical fields, and EMI effects associated with them. One conventional method of producing gas ionization is known as corona discharge. Corona-based ionizers, (see, for example, published patent applications US 20070006478, JP 2007048682) are desirable in that they may be energy and ionization efficient in a small space. However, one known drawback of such corona discharge apparatus is that the high voltage ionizing electrodes/emitters (in the form of sharp points or thin wires) generate undesirable contaminants along with the desired gas ions. Corona discharge may also stimulate the formation of tiny droplets of water vapor, for example, in the ambient air.
The formation of solid contaminant byproducts may also result from emitter surface erosion and/or chemical reactions associated with corona discharge in an ambient air/gas atmosphere. Surface erosion is the result of etching or spattering of emitter material during corona discharge. In particular, corona discharge creates oxidation reactions when electronegative gasses such as air are present in the corona. The result is corona byproducts in form of undesirable gases (such as ozone, and nitrogen oxides) and solid deposits at the tip of the emitters. For that reason conventional practice to diminish contaminant particle emission is to use emitters made from strongly corrosive-resistant materials. This approach, however, has its own drawback: it often requires the use of emitter material, such as tungsten, which is foreign to the technological process, such as semiconductor manufacturing. The preferred silicon emitters for ionizers used to neutralize charge during the manufacture of semiconductor silicon wafers do not possess the desired etching and corrosive resistance.
An alternative conventional method of reducing erosion and oxidation effects of emitters in corona ionizers is to continuously surround the emitter(s) with a gas flow stream/sheath of clean dry air (CDA), nitrogen, etc. flowing in the same direction as the main gas stream. This gas flow sheath is conventionally provided by gas source of gas as shown and described in published Japanese application JP 2006236763 and in U.S. Pat. No. 5,847,917.
U.S. Pat. No. 5,447,763 Silicon Ion Emitter Electrodes and U.S. Pat. No. 5,650,203 Silicon Ion Emitter Electrodes disclose relevant emitters and the entire contents of these patents are hereby incorporated by reference. To avoid oxidation of semiconductor wafers manufacturers utilize atmosphere of electropositive gasses like argon and nitrogen. Corona ionization is accompanied by contaminant particle generation in both cases and, in the latter case, emitter erosion is exacerbated by electron emission and electron bombardment. These particles move with the same stream of sheath gas and are able to contaminate objects of charge neutralization. Thus, in this context the cure for one problem actually creates another.
There are some important differences between an AC in-line ionizer and an AC or DC/pulsed DC ionizers operating in the ambient air or gas: single emitter of the in-line ionizer is isolated from ambient atmosphere (or gas) and there is no electrical field from a charged object to affect an ionization cell.
In contrast, ambient ionizer emitter(s) “see” electrical field from charged object and this field participates in ion clouds movement. Moreover, the emitter(s) in the ambient ionizer is not isolated from ambient atmosphere or gas. Consequently, in the ambient ionizer vacuum flow alone does not solve the problem of emitter contamination. In fact, vacuum flow inside an ionizer could create a dragging effect (sucking) for a portion of the ambient air which could, in turn, lead to the accumulation of a type of debris around the emitter point known as a “fuzz ball”.